Heparin-lnduced Thrombocytopenia (HIT) is a life-threatening thrombotic illness caused by drug-dependent antibodies directed to complexes of Platelet Factor 4 (PF4) and heparin. Although HIT, by definition, is a drug-induced disorder, its immunologic features bear little resemblance to other commonly encountered drug allergies, like quinine or penicillin. Unlike most drug-dependent immune responses, which are idiosyncratic and long-lived, the antibody response to heparin displays dose dependence, is short-lived and may lack immune memory. Based on these atypical features of the antibody response to drug, as well as known antibody specificities towards self-antigens (PF4 and endogenous glycosaminoglycans) in HIT, we posit that HIT is a T-cell dependent immune disorder in which self-tolerance is transiently dysregulated. Specifically, we hypothesize that HIT arises from a breach in self-tolerance brought about by immunostimulatory signals (involving antigenic PF4/heparin complexes and platelet CD154) and impaired immunoregulatory mechanisms. To test this hypothesis, we have developed a murine autoimmune model in which PF4/heparin antibodies (anti-mP+H) arise de novo after antigen challenge. In preliminary studies, we show that anti-mPF4/heparin from these animals recapitulate key aspects of the HIT immune response, including its variable incidence, serologic specificities and functional properties. We also show that mPF4/heparin antibody production in animals requires CD4+ T-helper cells and that immune recall is variably expressed in BALB/c and C57BL/6 mice. Using this animal model, we will: 1) Demonstrate cellular activation by macromolecular PF4/heparin complexes. In this aim, we will perform in vitro and in vivo studies to delineate lymphocyte responses to antigen and demonstrate that mPF4/heparin complexes trigger APC activation. 2) Delineate the role of platelet activation and platelet CD154 (Cp40L) in induction of the HIT immune response. Our preliminary studies indicate that in vivo platelet activation induces PF4/heparin antibody production. We hypothesize that platelet activation is essential for immune initiation in HIT, as activated platelets provide both a source of antigen (PF4) and co-stimulatory signals (CD154) needed for antibody production. To test this hypothesis, we will examine the sensitizing effects of platelet activation, mPF4 and platelet CD154 on the HIT immune response. 3) Delineate impaired immunoregulatory mechanisms in the murine immune response to PF4/heparin. Our preliminary studies show striking differences in patterns of immune recall in BALB/c and C57BL/6 mice that suggest differences in immunoregulatory function in animals. In this aim, we will test our hypothesis that regulatory T cells play a critical role in defining the course of immune response in HIT. It is expected that these studies will lead to insights into strategies that prevent or modulate immune complications from this life-threatening thrombotic disorder.